1. Field of the Invention
The present invention relates to a wiring substrate made of an inorganic material and a semiconductor package in which a semiconductor chip is mounted on the wiring substrate.
2. Description of the Related Art
Wiring substrates made of organic materials such as insulative resins and wiring substrates made of inorganic materials such as silicon are known. In wiring substrates made of organic materials, a planar power plane and ground (GND) plane are formed to attain impedance matching, reduction of power source impedance, etc., as disclosed in JP-A-2001-148448.
The resistance of a power pattern or a ground pattern can be made low by making the area of a power plane or a GND plane as wide as possible. When opposed to each other at a small interval, a power plane and a GND plane can be made more capacitive than inductive. Furthermore, impedance matching can be attained by interposing a signal electrode between a power plane and a GND plane.
On the other hand, in wiring substrates made of inorganic materials, because of a microloading effect etc., it is difficult to form a planar power plane or GND plane. Therefore, a power line and a GND line are each formed merely by very fine wiring patterns and a via line.
Besides, the microloading effect is a phenomenon that the etching rate varies depending on the aspect ratio (i.e., the ratio between the width and the depth) of a pattern.
However, if a power line and a GND line are each formed merely by very fine wiring patterns and a via line in a wiring substrate made of an inorganic material, layers having wide conductor areas cannot be secured, as a result of which the power line and the GND line are made large in resistance and become more capacitive than inductive. As a result, power source impedance is made high. Furthermore, impedance matching with a line for high-speed input/output signals cannot be attained.
Even if the structure disclosed in JP-A-2001-148448 is employed, it is not easy to attain impedance matching though low power source impedance can be attained.